The invention relates to an ultraviolet-resistant foam member which is used as a sound reducer in an exposure lighthouse. A number of methods are described for preparing such a member.
Exposure lighthouses that are used for producing mosaic-type viewing screens for cathode-ray tubes are described in the prior art; for example in U.S. Pat. Nos. 2,942,099 to N. R. Goldstein, 3,592,112 to H. R. Frey, 3,970,456 to T. W. Branton and 4,562,120 to R. H. Axelrod et al. Such lighthouses generally include a metal light box, a cooled, small-area lamp which emits both visible and ultraviolet light and opticl components for reflecting and refracting light from the lamp to a photosensitive layer on a support for a viewing screen. Except for the optical components, it is desirable that inner surfaces of the lighthouse carry a light-absorbent coating that reflects a minimum of light that is incident thereon in order to minimize light scattering. The lamp used in the lighthouse is strongly emissive at about 365 nanometers and has an output over the range of 200 to 750 nanometers. Additionally, the lamp generates a significant amount of heat such that some lighthouse parts reach temperatures above 700.degree. C. The lamp may be either air cooled or water cooled, and while air cooling provides the simplest mechanical system, such air cooled systems produce a characteristic hissing noise which must be controlled to be in compliance with OSHA regulations. Prior art foams, such as polyurethane and silicone rubber, have cell-like structures which can absorb or dampen sound waves and are suitable as noise reducers. However, the above-described foam materials craze and become brittle after about 24 hours of exposure to the ultraviolet radiation of the lamp. The UV degradated foams cannot effectively absorb noise and begin to decompose and produce foam particles. These particles are deposited, for example, on the lighthouse optical components resulting in defects in the viewing screen.